U.S. Pat. No. 8,958,490 titled “COFDM broadcasting with single-time retransmission of COFDM symbols” was granted 17 Feb. 2015 to Allen LeRoy Limberg. U.S. Pat. No. 8,958,490 describes COFDM receivers that decode the FEC coding of initial transmissions and of final transmissions for iterative-diversity reception separately; data packets without error or with as little error as available are then chosen from the results of such separate decoding. U.S. Pat. No. 8,958,490 also describes COFDM receivers that use the plural-input/single-output detectors used in the prior art for plural-antenna spatial-diversity reception, using such detectors for combining initial and final transmissions for iterative-diversity reception. These PISO detectors unmap the QAM of COFDM carriers in those initial and final transmissions, then decode the FEC coding of the unmapping results. Still further, U.S. Pat. No. 8,958,490 describes COFDM receivers that use maximal-ratio code combining of the coordinates of corresponding QAM symbol constellations from initial and final transmissions of similar COFDM signals to implement iterative-diversity reception. The maximal-ratio code combining is done before unmapping those constellations.
Delaying the final transmissions of coded DTV signals up to few seconds respective to the initial transmissions of them allows receivers of suitable design to tolerate deep full-spectrum fades that last up to a few seconds, as sometimes occur in received COFDM signals. Such full-spectrum fading is sometimes referred to as a “drop-out” in received signal strength. COFDM receivers in moving vehicles experience such drop-outs when the receiving site is changed such that a sole effective signal transmission path is blocked by an intervening hill or structure, for example. Because the signaling rate in the individual OFDM carriers is very low, COFDM receivers are capable of maintaining reception despite drop-outs that are only a fraction of a second in duration. However, drop-outs that last a second or more disrupt television reception perceptibly. Automatic gain control of the front-end tuner stages of a DTV receiver will increase their gain, amplifying noise to introduce burst noise into the FEC coding. Such protracted drop-outs are encountered in a vehicular receiver when the vehicle passes through a tunnel, for example. By way of further example of a protracted drop-out in reception, a stationary receiver may experience drop-outs in received signal strength during dynamic multipath reception conditions, such as caused by an aircraft flying over the reception site. Electric motors can generate radio-frequency noise strong enough to overload the front-end tuner stages of a DTV receiver, acting as a jamming signal that obliterates COFDM reception and generates burst noise too long to be corrected by FEC coding.
If a reception site is not more than a few kilometers distant from the COFDM transmitter or transmitters, multipath reception can cause severe frequency-selective fading of a large group of OFDM carriers in a particular portion of the radio-frequency (RF) channel. Frequency-selective fading has been observed that extends over three MHz in a 6-MHz-wide RF channel and reduces the amplitude of a central few of the selectively faded OFDM carriers as much as 35 dB respective to less affected OFDM carriers. Such frequency-selective fading corrupts so many of the QAM symbols used for modulating respective OFDM carriers that de-interleaving of the results of unmapping the QAM symbols is unable to reduce the density of bit errors in the recovered bit-wise FEC coding sufficiently to permit successful decoding thereof. In some circumstances a directional reception antenna may be able to mitigate this problem. However, the whip antenna of a hand-held receiver is apt not to have appreciable capability for rejecting co-channel interference.
U.S. Pat. No. 8,958,490 discloses the following procedures to overcome severe frequency-selective fading of a large group of OFDM carriers in a particular portion of the RF channel. COFDM symbols of initial transmissions of the coded DTV data are arranged such that their circular discrete Fourier transforms (DFTs) are rotated one-half revolution respective to the circular DFTs of corresponding COFDM symbols in time-slices of subsequent transmissions of that same DTV data. DTV receivers then de-rotate the COFDM symbols of initial transmissions of the DTV data and after delaying the resulting COFDM symbols combine them with COFDM symbols of subsequent transmissions of that same DTV data. Such DTV receivers are capable of overcoming severe frequency-selective fading that is apt to be caused by multipath reception from nearby DTV transmitters, as well as overcoming protracted severe flat-spectral fading of one of the initial and subsequent transmissions of the same DTV data. These DTV receivers also better tolerate narrow-band noise affecting only a small portion of the frequency spectrum of the radio-frequency channel.
Transmitting forward-error-correction (FEC) coded data twice separated by some time interval provides a COFDM receiver with additional basis for overcoming randomly occurring short-duration burst noise, besides relying on coding that corrects burst errors. However, retransmitting COFDM symbols without intervening delay, or with intervening delay of only a few OFDM symbol intervals, still facilitates receivers of suitable design better to overcome randomly occurring burst noise of short duration.
U.S. Pat. No. 9,143,375 titled “Iterative-diversity COFDM broadcasting with improved shaping gain” was granted 12 Sep. 2015 to Allen LeRoy Limberg and is incorporated herein by reference. U.S. Pat. No. 9,143,375 prescribes that the coded DTV signals of initial transmissions and of final transmissions be mapped to quadrature amplitude modulation (QAM) of the COFDM carriers according to first and second patterns, respectively. Bits that map to lattice points in the first mapping pattern more likely to experience error are mapped to lattice points in the second mapping pattern less likely to experience error. Bits that map to lattice points in the second mapping pattern more likely to experience error are mapped to lattice points in the first mapping pattern less likely to experience error. In this specification and the related drawings the two different mappings are referred to as “conjugate mappings” forming a “conjugate pair” of mappings to QAM symbol constellations. Receiver apparatus combines the earlier and later transmissions of twice-transmitted COFDM signals as part of iterative procedures for unmapping QAM and decoding the LDPC coding of each of the DTV signals. Such twice-transmitted COFDM signals cannot be maximal-ratio combined before unmapping the QAM symbols they respectively convey. U.S. Pat. No. 9,143,375 describes each of the twice-transmitted COFDM signals being unmapped, de-interleaved and decoded, with results of the decoding of forward-error-correction coding being selected between on the basis of less likelihood of error. This is a rather complex reception procedure, which can be avoided as described hereinafter.
U.S. Pat. No. 7,236,548 titled “Bit level diversity combining for COFDM system” was granted 26 Jun. 2007 to Monisha Ghosh, Joseph P. Meehan and Xuemei Ouyang. U.S. Pat. No. 7,236,548 describes a wireless communications receiver for similar COFDM signals received concurrently via different antenna elements. Bit level, rather than symbol level, maximal-ratio combining is employed subsequent to unmapping QAM symbol constellations. Combining at the QAM symbol level increases the effective SNR for an AWGN channel by 3 dB, whether done before or after unmapping QAM symbol constellations. U.S. Pat. No. 7,236,548 asserts that maximal-ratio combining at the bit level after unmapping QAM symbol constellations increases the effective SNR for the AWGN channel by an additional 2.5 dB or so when diversity combining the results of unmapping two similar QAM symbol constellations. U.S. Pat. No. 7,236,548 attributes this increase in the effective SNR for the AWGN channel to taking advantage of equalization that is embedded in the decoding process.